The present exemplary embodiments relate to phosphors for the conversion of radiation emitted by a light source. They find particular application in conjunction with converting LED-generated ultraviolet (UV), violet or blue radiation into white light for general illumination purposes. It should be appreciated, however, that the invention is also applicable to the conversion of radiation from UV, violet and/or blue lasers as well as other light sources to white light.
Light emitting diodes (LEDs) are semiconductor light emitters often used as a replacement for other light sources, such as incandescent lamps. They are particularly useful as display lights, warning lights and indicating lights or in other applications where colored light is desired. The color of light produced by an LED is dependent on the type of semiconductor material used in its manufacture.
Colored semiconductor light emitting devices, including light emitting diodes and lasers (both are generally referred to herein as LEDs), have been produced from Group III-V alloys such as gallium nitride (GaN). To form the LEDs, layers of the alloys are typically deposited epitaxially on a substrate, such as silicon carbide or sapphire, and may be doped with a variety of n and p type dopants to improve properties, such as light emission efficiency. With reference to the GaN-based LEDs, light is generally emitted in the UV and/or blue range of the electromagnetic spectrum. Until quite recently, LEDs have not been suitable for lighting uses where a bright white light is needed, due to the inherent color of the light produced by the LED.
Recently, techniques have been developed for converting the light emitted from LEDs to useful light for illumination purposes. In one technique, the LED is coated or covered with a phosphor layer. A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of very high chemical purity and of controlled composition to which small quantities of other elements (called “activators”) have been added to convert them into efficient fluorescent materials. With the right combination of activators and host inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range.
By interposing a phosphor excited by the radiation generated by the LED, light of a different wavelength, e.g., in the visible range of the spectrum, may be generated. Colored LEDs are often used in toys, indicator lights and other devices. Manufacturers are continuously looking for new colored phosphors for use in such LEDs to produce custom colors and higher luminosity.
In addition to colored LEDs, a combination of LED generated light and phosphor generated light may be used to produce white light. The most popular white LEDs are based on blue emitting GaInN chips. The blue emitting chips are coated with a phosphor that converts some of the blue radiation to a complementary color, e.g. a yellow-green emission. The total of the light from the phosphor and the LED chip provides a color point with corresponding color coordinates (x and y on the CIE chromaticity diagram) and correlated color temperature (CCT), and its spectral distribution provides a color rendering capability, measured by the color rendering index (CRI).
The CRI is commonly defined as a mean value for 8 standard color samples (R1-8), usually referred to as the General Color Rendering Index and abbreviated as Ra, although 14 standard color samples are specified internationally and one can calculate a broader CRI (R1-14) as their mean value. In particular, the R9 value, measuring the color rendering for the strong red, is very important for a range of applications, especially of medical nature. As used herein, “CRI” is used to refer to any of the above general, mean, or special values unless otherwise specified.
One known white light emitting device comprises a blue light-emitting LED having a peak emission wavelength in the blue range (from about 440 nm to about 480 nm) combined with a phosphor, such as cerium doped yttrium aluminum garnet Y3Al5O12: Ce3+ (“YAG”). The phosphor absorbs a portion of the radiation emitted from the LED and converts the absorbed radiation to a yellow-green light. The remainder of the blue light emitted by the LED is transmitted through the phosphor and is mixed with the yellow light emitted by the phosphor. A viewer perceives the mixture of blue and yellow light as a white light.
So far, it has been extremely difficult to fine-tune the CRI of a phosphor-converted white light LED around given color point and luminous efficacy targets. As detailed above, previously proposed methods of white LED manufacturing use either a single phosphor composition (containing 1 or more phosphor compounds), or a layered structure of phosphor compositions, each with a substantially different color point, providing a color balance.
In this respect, attention is directed to FIG. 1, which shows an exemplary phosphor conversion light emitting device 10 as shown. The light emitting device 10 comprises a semiconductor UV or blue radiation source, such as a light emitting diode (LED) chip or die 12 and leads 16, 18 electrically attached to the LED chip. The leads may comprise thin wires supported by a thicker lead frame(s) 14 or the leads may comprise self supported electrodes and the lead frame may be omitted. The leads 16, 18 provide current to the LED chip 12 and thus cause the LED chip 12 to emit radiation. The chip 12 is covered by a phosphor containing layer 20. The phosphor material utilized in the layer 20 can vary, depending upon the desired color of secondary light that will be generated by the layer 20. The chip 12 and the phosphor containing layer 20 are encapsulated by an encapsulant 22.
In operation, electrical power is supplied to the die 12 to activate it. When activated, the chip 12 emits the primary light away from its top surface. The emitted primary light is absorbed by the phosphor containing layer 20. The phosphor layer 20 then emits a secondary light, i.e., converted light having a longer peak wavelength, in response to absorption of the primary light. The secondary light is emitted randomly in various directions by the phosphor in the layer 20. Some of the secondary light is emitted away from the die 12, propagating through the encapsulant 22 and exiting the device 10 as output light. The encapsulant 22 directs the output light in a general direction indicated by arrow 24.
Both the single phosphor composition (containing 1 or more phosphor compounds) approach and the layered structure of phosphor compositions (each with a substantially different color point) approach, provides a given set of luminosity and CRI values which are fixed either by the chemical composition or the relative size of the phosphor layers, and cannot be changed further without either redesigning the phosphor blend or losing the color balance of the device.
It would therefore be desirable to develop new LED based solutions that allow tuning the CRI (e.g. maximizing for a given minimal luminosity requirement) or the luminosity (e.g. maximizing for a given minimal CRI requirement) without affecting the chemical composition of the phosphor blend or compromising the color point target. This affords a set of 2 basic phosphor compositions to be used for the manufacturing of white LEDs with the same color point but with CRI or luminosity values customized for specific applications. The present invention provides new and improved phosphor layering methods, blends and method of formation, which overcome the above-referenced problems and others.